Naturally occurring trona, such as found in subterranean deposits in Sweetwater and adjacent counties in Wyoming, consists mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O) containing about 3 to 15% or, more generally, 5 to 10% by weight of insoluble impurities, mainly shale. Since the trona is usually found near the oil shale formations of the upper Colorado River basin, it is further contaminated by carbonaceous matter, usually in amount in the order of about 0.1 to 0.5% by weight, expressed as elemental carbon.
In the production of sodium carbonate from natural trona, it is usual procedure to calcine the trona at elevated temperature, in the order of 200.degree. to 600.degree. C., to obtain crude sodium carbonate: EQU 2 (Na.sub.2 CO.sub.3.NaHCO.sub.3.2 H.sub.2 O).fwdarw. 3Na.sub.2 CO.sub.3 + 5H.sub.2 O+ CO.sub.2
calcination of the trona effects moisture removal, reduction or elimination of carbonaceous matter as well as transformation of the sodium sesquicarbonate into sodium carbonate. In general, elimination of organic matter is favorably influenced by increase in calcination temperature. However, calcination at elevated temperature also causes sodium carbonate to react with silica contained in the trona to form soluble silicates. The quantity of soluble silicates so formed increases materially with increase in calcination temperature.
Both, soluble carbonaceous matter as well as soluble silicates are major impurities in solutions obtained by dissolving calcined trona and, unless steps are taken to reduce contamination of sodium carbonate process liquors with soluble silicates and carbonaceous matter, these contaminants may become significant impurities in the soda ash prepared from such liquors. Carbonaceous matter, if not removed from the sodium carbonate process liquors, not only contaminates the soda ash product, but such carbonaceous matter also includes crystal habit modifiers which adversely modify the crystal structure of the sodium carbonate monohydrate produced by crystallization from the sodium carbonate process liquors, and further causes troublesome foaming of such liquors.
Two methods are commonly employed for eliminating undesirable carbonaceous matter in the trona soda ash process. The first method involves treatment of the trona process liquor with activated carbon. The major drawback of this approach is that not all of the critical organic crystal habit modifiers are readily absorbed and tend to remain in solution, causing less than ideal crystals to form. Furthermore, treatment of process liquors with activated carbon involves considerable expense and poses disposal problems for the spent carbon. The second method involves burning off the carbonaceous matter by conducting calcination at very high temperatures in excess of about 350.degree. C. This method is quite effective, but unfortunately causes increase in formation of soluble silicates.